At the point you've come to and the general condition of the framing, I'd suggest you look at pulling all of the inside paneling, rebuild the framing, and patch up the roof. The general aspect of the framing is not good, too many loose ends, unsupported framing members, and not built to withstand even moderate use. I'd guess that the quality of the paneling was a major contributor to keeping it relatively in one piece.
If you love propane lighting, keep the copper. I've had a trailer with gas lighting and found it to be a hassle. I had to buy new mantles for every trip. With the advent of LED lighting, IMO, there isn't a need for gas lighting.
The two areas that were discovered on the roof to be cracked, looks like a stress crack. Movement and the curve of the roof probably developed metal fatigue. Patch and support of that area will be the cure.
We don't know your budget or time commitment for this project but my 22' Starcraft was completed for < $5K and about 6 mo. of dedicated work. That was a complete restoration with greatly upgraded insulation, windows, and wiring. YMMV, but I think you are approaching total restoration commitment turf.
